1. Field of the Invention
The present invention relates to an apparatus for automatically measuring a blood pressure of a living subject according to so-called oscillometric method.
2. Related Art Statement
The oscillometric blood-pressure measuring method is carried out as follows: A pressing pressure of an inflatable cuff which is wound around, e.g., an upper arm of a living subject, that is, a cuff pressure is slowly changed, and a blood pressure of the subject is determined based on the change of an oscillatory component occurring to the cuff during the changing of the cuff pressure (i.e., the change of respective amplitudes of respective heartbeat-synchronous pulses of a cuff pulse wave with respect to the change of cuff pressure). An automatic blood-pressure measuring apparatus which employs the oscillometric method can measure a blood pressure at a front neck, a finger, or an inferior limb of a living subject where it is difficult to measure a blood pressure according to a microphone-using method. In addition, the microphone-using method requires a microphone to be accurately positioned on an artery of the subject, whereas the oscillometric method just needs the cuff to be wound around a portion of the subject. Thus, the oscillometric method can be easily carried out by both skilled and non-skilled operators. Moreover, the oscillometric method can be carried out at a noisy place, or on a child or an in-shock patient who produces small Korotkoff sounds only. Thus, the oscillometric-type automatic blood-pressure measuring devices have come into wide use.
However, according to the oscillometric method, a blood pressure is determined based on the change of respective amplitudes of respective pulses of the cuff pulse wave occurring to the cuff during the slow changing of cuff pressure, on the assumption that the blood pressure of the subject does not change during the changing of cuff pressure. More specifically described, it is assumed that respective amplitudes A, and respective prescribed points, such as minimal values (i.e., diastolic pressures) or maximal values (i.e., systolic pressures), of respective pulses of a blood-pressure wave, shown in FIG. 1A, do not change during the changing of cuff pressure.
However, in an actual blood-pressure measurement, the cardiac output of the subject may change during the changing of cuff pressure, so that as shown in FIG. 1B, the respective amplitudes A of respective pulses of the blood-pressure wave may change, or so that as shown in FIG. 1C, the amplitudes A may not change but the blood pressure values may change. In some cases, both the amplitudes A and the blood pressure values may change. If the cardiac output or the blood pressure largely changes during the blood-pressure measurement, the envelope of respective amplitudes of respective pulses of the cuff pulse wave with respect to the cuff pressure deforms, which leads to determining an inaccurate blood pressure or even failing to determine a blood pressure.
It is therefore an object of the present invention to provide an apparatus which automatically measures a blood pressure of a living subject with high accuracy.
The Inventor has carried out extensive studies and found the following facts: When amplitude of a blood-pressure waveform changes, amplitude of a volumetric pulse wave also changes. Therefore, if the same corrections to correct respective amplitudes of a volumetric pulse wave obtained during the changing of pressure of an inflatable cuff, to a predetermined value, are applied to respective amplitudes of a cuff pulse wave occurring to the cuff during the changing of the cuff pressure, an accurate blood pressure can be determined based on the thus corrected amplitudes of the cuff pulse wave. In addition, when blood pressure changes, pulse period also changes. Therefore, if blood pressure values are estimated based on pulse period values obtained during the changing of the cuff pressure, and the same corrections to correct the respective blood pressure values to a predetermined value, are applied to respective values of the cuff pressure at respective times of detection of the pulse period values, an accurate blood pressure can be determined based on the thus corrected values of the cuff pressure. Moreover, if those two inventions are combined, a more accurate blood pressure can be determined. Moreover, if a pulse pressure (i.e., an amplitude of a blood-pressure waveform) is estimated, based on two successive pulse period values obtained during the changing of the cuff pressure, according to a known technique, and the same corrections to correct respective pulse pressure values, determined based on respective pairs of successive pulse period values obtained during the changing of the cuff pressure, to a predetermined value, are applied to respective amplitudes of a cuff pulse wave occurring to the cuff during the changing of the cuff pressure, an accurate blood pressure can be determined based on the thus corrected amplitudes of the cuff pulse wave. Furthermore, if this invention is combined with the above-indicated second invention, a more accurate blood pressure can be determined.
The above object has been achieved by the present invention. According to the first invention, there is provided an apparatus for automatically measuring a blood pressure of a living subject, comprising an inflatable cuff which is adapted to be wound around a first portion of the subject, a cuff pulse wave including a plurality of heartbeat-synchronous pulses occurring to the cuff when a pressure in the cuff is changed; a volumetric-pulse-wave detecting device which is adapted to be worn on a second portion of the subject and which detects, from the second portion of the subject, a volumetric pulse wave including a plurality of heartbeat-synchronous pulses; a correction-factor determining means for iteratively determining, for each of the heartbeat-synchronous pulses of the volumetric pulse wave, a correction factor to correct an amplitude of the each heartbeat-synchronous pulse to a predetermined value; a corrected-cuff-pulse-wave-amplitude determining means for iteratively multiplying, by the correction factor determined by the correction-factor determining means for the each of the heartbeat-synchronous pulses of the volumetric pulse wave, an amplitude of one of the heartbeat-synchronous pulses of the cuff pulse wave that corresponds to the each heartbeat-synchronous pulse of the volumetric pulse wave, and thereby iteratively determining a corrected amplitude of the one heartbeat-synchronous pulse of the cuff pulse wave; and a blood-pressure determining means for determining a blood pressure of the subject based on a change of the respective corrected amplitudes of the respective heartbeat-synchronous pulses of the cuff pulse wave, with respect to the pressure of the cuff.
According to this invention, the correction-factor determining means iteratively determines, for each of the pulses of the volumetric pulse wave detected by the volumetric-pulse-wave detecting device, a correction factor to correct an amplitude of the each pulse to a predetermined value, and the corrected-cuff-pulse-wave-amplitude determining means iteratively multiplies, by the correction factor determined for the each of the pulses of the volumetric pulse wave, an amplitude of one of the pulses of the cuff pulse wave that corresponds to the each pulse of the volumetric pulse wave, and thereby iteratively determines a corrected amplitude of the one pulse of the cuff pulse wave. The thus determined corrected cuff-pulse-wave amplitudes are free of the influence of change of cardiac output of the subject during the changing of pressure of the cuff. And, the blood-pressure determining means determines a blood pressure of the subject based on a change of the respective corrected amplitudes of the respective pulses of the cuff pulse wave, with respect to the pressure of the cuff. Therefore, the thus determined blood pressure enjoys a high accuracy.
According to the second invention, there is provided an apparatus for automatically measuring a blood pressure of a living subject, comprising an inflatable cuff which is adapted to be wound around a portion of the subject, a cuff pulse wave including a plurality of heartbeat-synchronous pulses occurring to the cuff when a pressure in the cuff is changed; a heartbeat-synchronous-wave detecting device which detects, from the subject, a heartbeat-synchronous wave including a plurality of heartbeat-synchronous pulses; a pulse-period determining means for iteratively determining a pulse period of the subject based on a time interval between respective prescribed periodic points of each pair of successive heartbeat-synchronous pulses of the heartbeat-synchronous wave detected by the heartbeat-synchronous-wave detecting device during the changing of the pressure of the cuff an estimated-blood-pressure determining means for iteratively determining an estimated blood pressure of the subject during the changing of the pressure of the cuff, based on each of the pulse periods iteratively determined by the pulse-period determining means, according to a predetermined relationship between estimated blood pressure and pulse period; a blood-pressure-correction-value determining means for iteratively determining, for the each of the pulse periods, a blood-pressure correction value to correct a corresponding one of the estimated blood pressures iteratively determined by the estimated-blood-pressure determining means, to a predetermined value; a corrected-cuff-pressure determining means for subtracting, from a pressure of the cuff at a time of detection of one of the heartbeat-synchronous pulses of the cuff pulse wave that corresponds to the each of the pulse periods, the blood-pressure correction value determined by the blood-pressure-correction-value determining means for the each of the pulse periods, and thereby determining a corrected pressure of the cuff for the each of the pulse periods; and a blood-pressure determining means for determining a blood pressure of the subject based on a change of respective amplitudes of the respective heartbeat-synchronous pulses of the cuff pulse wave, with respect to the respective corrected pressures of the cuff determined by the corrected-cuff-pressure determining means for the respective pulse periods.
According to this invention, the estimated-blood-pressure determining means iteratively determines an estimated blood pressure of the subject during the changing of the pressure of the cuff, based on each of the pulse periods, and the blood-pressure-correction-value determining means iteratively determines, for the each of the pulse periods, a blood-pressure correction value to correct a corresponding one of the estimated blood pressures, to a predetermined value. The corrected-cuff-pressure determining means subtracts, from a pressure of the cuff at a time of detection of one of the pulses of the cuff pulse wave that corresponds to the each of the pulse periods, the blood-pressure correction value determined for the each of the pulse periods, and thereby determines a corrected pressure of the cuff for the each of the pulse periods. The thus determined corrected pressure values of the cuff would be detected at respective times of detection of respective pulses of the cuff pulse wave in a state in which there would be no influence of change of blood pressure of the subject during the changing of the cuff pressure. And, the blood-pressure determining means determines a blood pressure of the subject based on a change of respective amplitudes of the respective pulses of the cuff pulse wave, with respect to the respective corrected pressure values of the cuff determined by the corrected-cuff-pressure determining means. Therefore, the thus determined blood pressure enjoys a high accuracy.
According to the third invention, there is provided an apparatus for automatically measuring a blood pressure of a living subject, comprising an inflatable cuff which is adapted to be wound around a first portion of the subject, a cuff pulse wave including a plurality of heartbeat-synchronous pulses occurring to the cuff when a pressure in the cuff is changed; a volumetric-pulse-wave detecting device which is adapted to be worn on a second portion of the subject and which detects, from the second portion of the subject, a volumetric pulse wave including a plurality of heartbeat-synchronous pulses; a correction-factor determining means for iteratively determining, for each of the heartbeat-synchronous pulses of the volumetric pulse wave, a correction factor to correct an amplitude of the each heartbeat-synchronous pulse to a predetermined value; a corrected-cuff-pulse-wave-amplitude determining means for iteratively multiplying, by the correction factor determined by the correction-factor determining means for the each of the heartbeat-synchronous pulses of the volumetric pulse wave, an amplitude of one of the heartbeat-synchronous pulses of the cuff pulse wave that corresponds to the each heartbeat-synchronous pulse of the volumetric pulse wave, and thereby iteratively determining a corrected amplitude of the one heartbeat-synchronous pulse of the cuff pulse wave; a heartbeat-synchronous-wave detecting device which detects, from the subject, a heartbeat-synchronous wave including a plurality of heartbeat-synchronous pulses; a pulse-period determining means for iteratively determining a pulse period of the subject based on a time interval between respective prescribed periodic points of each pair of successive heartbeat-synchronous pulses of the heartbeat-synchronous wave detected by the heartbeat-synchronous-wave detecting device during the changing of the pressure of the cuff, an estimated-blood-pressure determining means for iteratively determining an estimated blood pressure of the subject during the changing of the pressure of the cuff, based on each of the pulse periods iteratively determined by the pulse-period determining means, according to a predetermined relationship between estimated blood pressure and pulse period; a blood-pressure-correction-value determining means for iteratively determining, for the each of the pulse periods, a blood-pressure correction value to correct a corresponding one of the estimated blood pressures iteratively determined by the estimated-blood-pressure determining means, to a predetermined value; a corrected-cuff-pressure determining means for subtracting, from a pressure of the cuff at a time of detection of one of the heartbeat-synchronous pulses of the cuff pulse wave that corresponds to the each of the pulse periods, the blood-pressure correction value determined by the blood-pressure-correction-value determining means for the each of the pulse periods, and thereby determining a corrected pressure of the cuff for the each of the pulse periods; and a blood-pressure determining means for determining a blood pressure of the subject based on a change of the respective corrected amplitudes of the respective heartbeat-synchronous pulses of the cuff pulse wave determined by the corrected-cuff-pulse-wave-amplitude determining means, with respect to the respective corrected pressures of the cuff determined by the corrected-cuff-pressure determining means for the respective pulse periods.
According to this invention, the blood-pressure determining means determines a blood pressure of the subject based on a change of the respective corrected amplitudes of the respective pulses of the cuff pulse wave, with respect to the respective corrected pressures of the cuff. Since the corrected pressure values of the cuff would be detected at respective times of detection of respective pulses of the cuff pulse wave in a state in which there would be no influence of change of blood pressure of the subject during the changing of the cuff pressure, and the corrected cuff-pulse-wave amplitudes are free of the influence of change of cardiac output of the subject during the changing of pressure of the cuff, the determined blood pressure enjoys a higher accuracy.
According to the fourth invention, there is provided an apparatus for automatically measuring a blood pressure of a living subject, comprising an inflatable cuff which is adapted to be wound around a portion of the subject, a cuff pulse wave including a plurality of heartbeat-synchronous pulses occurring to the cuff when a pressure in the cuff is changed; a heartbeat-synchronous-wave detecting device which detects, from the subject, a heartbeat-synchronous wave including a plurality of heartbeat-synchronous pulses; a pulse-period determining means for iteratively determining a pulse period of the subject based on a time interval between respective prescribed periodic points of each pair of successive heartbeat-synchronous pulses of the heartbeat-synchronous wave detected by the heartbeat-synchronous-wave detecting device during the changing of the pressure of the cuff; an estimated-pulse-pressure determining means for iteratively determining, for each of the pulse periods iteratively determined by the pulse-period determining means, an estimated pulse pressure of the subject during the changing of the pressure of the cuff, based on each pair of successive pulse periods of the pulse periods, according to a predetermined relationship between pulse pressure and a pair of successive pulse periods; a correction-factor determining means for iteratively determining, for each of the estimated pulse pressures iteratively determined by the estimated pulse pressure determining means, a correction factor to correct the each estimated pulse pressure to a predetermined value; a corrected-cuff-pulse-wave-amplitude determining means for iteratively multiplying, by the correction factor determined by the correction-factor determining means for the each of the estimated pulse pressures, an amplitude of one of the heartbeat-synchronous pulses of the cuff pulse wave that corresponds to the each of the estimated pulse pressures, and thereby iteratively determining a corrected amplitude of the one heartbeat-synchronous pulse of the cuff pulse wave; and a blood-pressure determining means for determining a blood pressure of the subject based on a change of the respective corrected amplitudes of the respective heartbeat-synchronous pulses of the cuff pulse wave, with respect to the pressure of the cuff.
According to this invention, the correction-factor determining means iteratively determines, for each of the estimated pulse pressures iteratively determined by the estimated pulse pressure determining means, a correction factor to correct the each estimated pulse pressure to a predetermined value, and the corrected-cuff-pulse-wave-amplitude determining means iteratively multiplies, by the correction factor determined for the each of the estimated pulse pressures, an amplitude of one of the pulses of the cuff pulse wave that corresponds to the each of the estimated pulse pressures, and thereby iteratively determines a corrected amplitude of the one pulse of the cuff pulse wave. The thus determined corrected cuff-pulse-wave amplitudes are free of the influence of change of cardiac output of the subject during the changing of pressure of the cuff. And, the blood-pressure determining means determines a blood pressure of the subject based on a change of the respective corrected amplitudes of the respective pulses of the cuff pulse wave, with respect to the pressure of the cuff. Therefore, the determined blood pressure enjoys a higher accuracy.
According to the fifth invention, there is provided an apparatus for automatically measuring a blood pressure of a living subject, comprising an inflatable cuff which is adapted to be wound around a first portion of the subject, a cuff pulse wave including a plurality of heartbeat-synchronous pulses occurring to the cuff when a pressure in the cuff is changed; a heartbeat-synchronous-wave detecting device which detects, from the subject, a heartbeat-synchronous wave including a plurality of heartbeat-synchronous pulses; a pulse-period determining means for iteratively determining a pulse period of the subject based on a time interval between respective prescribed periodic points of each pair of successive heartbeat-synchronous pulses of the heartbeat-synchronous wave detected by the heartbeat-synchronous-wave detecting device during the changing of the pressure of the cuff, an estimated-pulse-pressure determining means for iteratively determining, for each of the pulse periods iteratively determined by the pulse-period determining means, an estimated pulse pressure of the subject during the changing of the pressure of the cuff, based on each pair of successive pulse periods of the pulse periods, according to a predetermined relationship between pulse pressure and a pair of successive pulse periods; a correction-factor determining means for iteratively determining, for each of the estimated pulse pressures iteratively determined by the estimated pulse pressure determining means, a correction factor to correct the each estimated pulse pressure, to a predetermined value; a corrected-cuff-pulse-wave-amplitude determining means for iteratively multiplying, by the correction factor determined by the correction-factor determining means for the each of the estimated pulse pressures, an amplitude of one of the heartbeat-synchronous pulses of the cuff pulse wave that corresponds to the each of the estimated pulse pressures, and thereby iteratively determining a corrected amplitude of the one heartbeat-synchronous pulse of the cuff pulse wave; an estimated-blood-pressure determining means for iteratively determining an estimated blood pressure of the subject during the changing of the pressure of the cuff, based on the each of the pulse periods iteratively determined by the pulse-period determining means, according to a predetermined relationship between estimated blood pressure and pulse period; a blood-pressure-correction-value determining means for iteratively determining, for the each of the pulse periods, a blood-pressure correction value to correct a corresponding one of the estimated blood pressures iteratively determined by the estimated-blood-pressure determining means, to a predetermined value; a corrected-cuff-pressure determining means for subtracting, from a pressure of the cuff at a time of detection of one of the heartbeat-synchronous pulses of the cuff pulse wave that corresponds to the each of the pulse periods, the blood-pressure correction value determined by the blood-pressure-correction-value determining means for the each of the pulse periods, and thereby determining a corrected pressure of the cuff for the each of the pulse periods; and a blood-pressure determining means for determining a blood pressure of the subject based on a change of the respective corrected amplitudes of the respective heartbeat-synchronous pulses of the cuff pulse wave determined by the corrected-cuff-pulse-wave-amplitude determining means, with respect to the respective corrected pressures of the cuff determined-by the corrected-cuff-pressure determining means for the respective pulse periods.
According to this invention, the blood-pressure determining means determines a blood pressure of the subject based on a change of the respective corrected amplitudes of the respective pulses of the cuff pulse wave, with respect to the respective corrected pressures of the cuff. Since the corrected pressure values of the cuff would be detected at respective times of detection of respective pulses of the cuff pulse wave in a state in which there would be no influence of change of blood pressure of the subject during the changing of the cuff pressure, and the corrected cuff-pulse-wave amplitudes are free of the influence of change of cardiac output of the subject during the changing of pressure of the cuff, the determined blood pressure enjoys a higher accuracy.